Self-priming centrifugal pump

ABSTRACT

A self-priming centrifugal pump having suction and separation chambers defined within a casing, and a rotatable impeller positioned in a volute for transferring fluid from the suction chamber to the separation chamber. Reprime lift capability and priming rate are substantially improved by an elbow-shaped priming passage communicating the lower region of the separation chamber with the lower region of the volute, and by an upstanding ramp-like shoulder formed in the volute below the impeller. The improved priming passage configuration and the ramp-like shoulder are believed to effect a more efficient delivery of fluid from the separation chamber to the impeller during pump priming, and to effect a more turbulent mixing in the impeller of fluid with air from the suction chamber whereby air is efficiently entrained in the exhausted fluid and the pump is rapidly reprimed.

United States Patent Meister et a1.

[ SELF-PRIMING CENTRIFUGAL PUMP [75] Inventors: David L. Meister; Edward L. Ohler;

Frank D. Makowski, Jr., all of Mansfield, Ohio [73] Assignee: Gorman-Rupp Company, Mansfield,

Ohio

[22] Filed: Oct. 31, 1973 [21] Appl. No.: 411,262

[52] US. Cl. A. 415/53 [51] Int. Cl. F04c1 5/00 [58] Field of Search 415/53 [56] References Cited UNITED STATES PATENTS 2,580,347 12/1951 Eggleston 4l5/53 2,627,817 2/1953 Mann et a1 415/53 3,072,063 l/l963 McFarlin a 415/53 3,279,386 [0/1966 Rupp et a1. 415/53 3,322,071 5/1967 Teter 415/53 Q unllll llllllll-l Aug. 5, 1975 [5 7] ABSTRACT A self-priming centrifugal pump having suction and separation chambers defined within a casing. and a ro tatable impeller positioned in a volute for transferring fluid from the suction chamber to the separation chamber Reprime lift capability and priming rate are substantially improved by an elbow-shaped priming passage communicating the lower region of the separation chamber with the lower region of the volutc, and by an upstanding ramplike shoulder formed in the volute below the impeller. The improved priming passage configuration and the ramp-like shoulder are believed to effect a more efi'lcient delivery of fluid from the separation chamber to the impeller during pump priming, and to effect a more turbulent mixing in the impeller of fluid with air from the suction chamber whereby air is efficiently entrained in the exhausted fluid and the pump is rapidly reprimed,

18 Claims, 3 Drawing Figures PATENTEU AUG 1975 SHEET AUG 5 I975 PATENTEU 9 1314 SHEET 2 PATENTED AUG 5 1915 F? 8 9 8,014

sum 3 SELF-PRIM1NG CENTRIFUGAL PUMP CROSQ l TERENCE TO RELATED PATENTS Cross reference is made to the following patents, the disclosures of which are incorporated herein by reference:

CENTRIFUGAL PUMP, US. Pat. No. 3,279,386 issued Oct. I8, 1966 to W. E. Rupp; S. B. McFarlin; and C. E. Young, Jr.

AIR RELEASE VALVE FOR SELF-PRlMlNG CENTRIFUGAL PUMP, US. Pat. No. 3,575,52l, issued Apr. 20, i971 to R. 1. Porter and S. B. McFarlin.

BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates generally to centrifugal pumps, and more particularly to an improved self priming centrifugal pump of the type designed to handle large solids encountered in domestic and industrial waste water and sewage systems.

2. Prior Art Centrifugal self-priming pumps having a capacity to handle fluids containing relatively large solids are known, as illustrated by the referenced patents. These pumps typically include a casing which defines separate suction and separation chambers. A rotatable impeller positioned in a volute is arranged to transfer fluid from the suction chamber to the separation chamber.

A suction check valve is normally provided adjacent the inlet to the suction chamber to retain fluid in the pump when the pump is shut down. If the check valve does not seat completely due to damage or due to de bris trapped in the valve, much of the fluid which has entered the pump is siphoned back out of the pump. The fluid backflow continues until the siphon leg is broken in the pump. The fluid which remains in the pump has a much lower level than results when the check valve seats properly. This lower level is defined in the industry as the pump reprime level.

Pump installation specifications typically specify that a pump be provided which can achieve a designated suction lift within a specified period of time with pump operation initiated at the reprime level. If a pump which has the required capacity to handle normal pumping requirements will not effect the required lift within the specified time starting at reprime level, it has been necessary to drive the pump at a higher speed to meet the reprime level starting specifications even though the larger pump capacity is not required once the pump is primed. The higher drive speed requires a higher power input which in turn requires a larger drive motor and heavier drive system components, etc.

The problem of providing a centrifugal pump with a self-priming capability is vastly magnified where the pump must be capable of handling fluids containing large solids. All openings through which the fluids flow must be large enough to pass the solids. Consequently, self-priming systems of the type employing small passages, restricted orifices, and the like, cannot be used.

An accepted and successful approach to providing a self-priming system for a solids-handling centrifugal pump is illustrated in the referenced CENTRIFUGAL PUMP patent. A priming opening communicates lower regions of the separation chamber and the volute. During priming, fluid is drawn through this opening from the separation chamber into the pump impeller where it mixes with air from the suction chamber.

No great significance has previously been attributed to the shape of the priming passage which communicates the separation chamber and the volute, or to the shape of lower volute region, and it has not previously been appreciated that the shape of these structures can be vastly improved to achieve a substantially higher reprime lift capablity and a much faster prime rate.

SUMMARY OF THE INVENTION The present invention relates to an improved selfpriming centrifugal pump capable of achieving substantially improved suction lift in substantially less time than has been possible with known self-priming centrifugal pumps.

Pumps constructed in accordance with the present invention have been shown to exhibit vastly improved reprime capabilities. By way of illustration, a 4-inch pump constructed in accordance with the referenced CENTRIFUGAL PUMP patent will effect a reprime lift of about 19 feet in 5 minutes time when driven at a speed of i750 RPM. An identical pump provided with the improvements of the present invention has been shown in tests to effect a reprime lift of about 26 feet in l minute time when driven at the same speed. Corresponding improvements in reprime lift and priming rate occur across the full range of pump operating speeds.

The improvements of the present invention relate to an improved configuration of the priming passage which communicates lower regions of the separation chamber and the volute, and to an improved configuration of the lower volute region. These improvements cooperate to substantially improve the self-priming ca pability of the pump.

In accordance with one feature of the present invention, the priming passage is defined by an elbow-shaped conduit having an inlet opening in the separation chamber and an outlet opening in the volute. The conduit defines a non-restricted passage of sufficient size to pass such solids as are present in the fluid.

In accordance with another feature of the present invention, the lower region of the pump volute is provided with a ramp-like shoulder which directs fluid entering from the priming passage upwardly and into the impeller. The presence of this shoulder has been found to substantially improve pump reprime performance. It is believed the shoulder contributes to the development and maintenance of a turbulent air-fluid mixing action in the pump impeller during pump priming, whereby air is more efficiently evacuated from the suction chamber than has been possible with prior pump designs.

Still another feature of the present invention relates to the orientation of the priming passage inlet opening. Fluids in the separation chamber normally circulate along a flow path which corresponds to the direction of fluid travel established as fluid is impelled through the volute. In accordance with the present invention, the priming passage inlet opening is positioned beside the normal fluid flow path in the separation chamber and is oriented such that fluid entering the inlet opening from the flow path must at least partially reverse their direction of flow. Preferably, the inlet opening is oriented to face opposite the direction of flow whereby entering fluids must effect a full l direction reversal to enter the reprime passage.

The 180 entering direction change combines with the turn encountered by the fluids in traversing the reprime passage, and with the upward turn of about 90 as the fluids enter the impeller to provide a full 360 of tortuous turns. It is believed that the effect of this tortuous flow path is to reduce the pressure of the fluid which enters the impeller. whereby a more continuous and larger flow of fluid is delivered to the impeller and an improved pump-priming performance results.

As will be apparent from the foregoing summary, it is a general object of the present invention to provide a novel and improved self-priming centrifgual pump.

Other objects and a fuller understanding of the invention may be had by referring to the following description and claims taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of an improved cen trifugal pump constructed in accordance with the present invention;

FIG. 2 is a cross sectional view on a reduced scale as seen from the plane indicated by the line 22 in FIG. I; and,

FIG. 3 is a perspective view on an enlarged scale of lower portions of the pump, upper portions being broken away to permit several features of the present invention to be illustrated with greater clarity.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, a self-priming centrifugal pump is shown generally at 10. The pump includes a casing 11 having a front wall 12, a rear wall 13, a partition wall 14, a top wall 15, and a bottom wall 16.

The partition wall 14 extends generally vertically and cooperates with the other casing walls to define separate side-by side suction and separation chambers l7, 18. Lower portions of the partition wall 14 form a wall 19 which is arcuate in transverse cross section. As best seen in FIG. 2. the wall 19 circumferentially defines a volute 20. A reinforcing strut ZI extends through the separation chamber [8 and connects the circumferential wall 19 to the rear wall 13.

An inlet fitting 22 is secured to the front wall 12 and communicates with the suction chamber 17. A check valve 23 is provided adjacent the inlet fitting to prevent the backflow of fluid from the suction chamber 17. The check valve 23 normally seats against an annular surface 24 formed on the inlet fitting 22. An outlet fitting 25 is secured to the top wall l5 and communicates with the separation chamber 18.

A rotatable impeller 26 is positioned in the volute 20 and is mounted on a drive shaft 27. The impeller drive shaft 27 is journaled for rotation by an assembly 30 which is removably carried in the casing 11. The assembly 30 includes a generally cylindrical housing 31 having front and rear radially extending mounting flanges 32, 33. Front and rear ball bearings 34, 35 are carried in the housing 31 and journal the drive shaft for rotation. The flanges 32. 33 have circumferentially extending grooves formed therein which carry O-rings 36, 37. Axially aligned openings 38, 39 formed in the cireum ferential wall [9 and the rear wall 13, respectively, receive the flanges 32, 33 and engage the O-rings 36, 37 to effect a fluid tight seal between the walls l9, l3 and the flanges 32, 33.

Axially aligned openings 40, 41 are formed through the front wall 12 and through forward portions of the circumferential wall 19 to permit the impeller 26 and its bearing assembly 30 to be installed as an assembled unit in the casing 11. A disc-shaped cover 42 closes the opening 40. Spaced projections 43 formed integrally with the cover 42 extend through the suction chamber 17 toward the impeller 26. An annular ring assembly 44 is secured by threaded fasteners 45 to the projections 43. The ring assembly 44 fits snugly within the opening 41 and provides a central opening 46 for admitting fluid from the suction chamber 17 to the impeller 26.

A threaded drain opening 50 is provided in the lower region of the front wall 12. A drain plug 51 is threaded into the opening 50. A drain passage 52 extends from the threaded opening 50 to an opening 53 which com municates with the volute 20.

The improvements of the present invention relate to the configuration ofa priming passage which communicates the lower region of the separation chamber 18 with the lower region of the volute 20. Referring to FIG. 3, an elbow-shaped conduit is cast integrally with the bottom wall 16 and the circumferential wall 19. The conduit extends from an inlet opening 61 in the separation chamber 18, to an outlet opening 63 in the volute 20. The conduit 60 preferably tapers upwardly slightly from the region of the inlet opening 6], as is best seen in FIG. I, and provides a non-restricted channel including a 90 turn which directs fluid into a sump 62 formed in the lower region of the volute 20.

A raised. ramp-like shoulder is provided in the lower region of the volute 20 at a position near to and spaced slightly downstream from the outlet opening 63 adjacent one side of the sump 62. While the shoulder 65 can extend substantially vertically, it is preferably inclined from the vertical, as indicated in FIG. 2 by the angle A," within the range of about 5 to about 35. The preferred range is about 15 to 30, with the most preferred angle of inclination being in the range of about 20 to about 25 from the vertical. The shoulder 65 is believed to assist in directing fluids from the sump 62 upwardly and into the region of operation of the impeller 26 during pump priming. It has been found that the shoulder 65 does not interfere with hydraulic flow in the volute 20 during normal (fully primed) pump operation, and that it does add measurably to reprime performance of the pump.

In operation. fluid containing solids is drawn into the suction chamber l7 through the inlet fitting 22. As long as the pump is in operation, the check valve 23 remains open to admit fluids to the suction chamber 17. The suction chamber fluids pass between the projections 43 and through the central opening 46 into the impeller 26.

As the impeller rotoates. the fluids circulate through the volute 20 as indicated by the arrows and discharge into the separation chamber 18. After the pump has reached an equilibrium operating condition, most of the fluid is discharged directly through the outlet fitting 25. Some fluid circulation occurs in the separation chamber 18, as indicated by the arrows 71, 72.

When the pump 10 is shut down, the check valve 23 normally seats against the surface 24 and prevents backflow of fluid from the suction chamber 17. If the valve 23 fails to seat properly due to damage or to the presence of debris across the seating surface 24, fluid will backflow from the suction chamber [7 through the inlet fitting 22. This backflow or siphoning action will continue until the siphon leg is broken in the pump.

The level of the fluid remaining in the pump is known as the pump reprime level.

When the pump is started from its reprime level, there is air in the suction chamber 17 and in the conduit (not shown) connected to the inlet fitting 22. This air must be evacuated and a new supply of fluid drawn in to reprime the pump before the pump will operate at full capacity.

The improvements of the present invention facilitate the rapid evacuation of air from the suction chamber so that pump priming takes place in minimal time. Fluid from the separation chamber 18 is drawn through the conduit 60 and directed upwardly by the shoulder 65 into the impeller. The tortuous flow path followed by the fluid in entering the conduit 60, traversing the length of the conduit, and turning upwardly over the shoulder 65 into the impeller is believed to cause the fluid entering the impeller to have a lower presure than is achieved in pumps constructed in accordance with the referenced CENTRIFUGAL PUMP patent. This lower pressure of entering pump fluid is believed to draw more fluid into the impeller and to provide a better mixing of fluid with air from the suction chamber.

The mixed air and fluid is then exhausted from the volute into the separation chamber 18. The exhausted fluid separates from the entrained air and travels back into the lower region of the separation chamber along the path indicated by the arrows 71. The fluid then reenters the conduit 60 and recirculates through the im' peller 26 to mix with and pump more air out of the suction chamber 17. This recirculation continues to effect a rapid evacuation of air from the suction chamber 17, whereby a new supply of fluid is drawn in through the inlet fitting 22 and the pump reprimes.

Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

1. A self-priming centrifugal pump capable of handling fluid containing solids, comprising:

a. a casing;

b. a volute in said casing and having a peripheral discharge opening in its upper portion, a priming opening in its lower portion, and a suction opening;

c. a rotatable impeller positioned in said volute and being operable when rotated to draw fluid through said suction opening and discharge it through said discharge opening;

d. a priming passage communicating with said priming opening and defining a conduit which requires fluid passing therethrough to change its direction of flow before entering said volute; and,

e. an upstanding ramp-like shoulder formed in the lower region of said volute below said impeller adjacent to and downstream from said priming opening for directing fluid entering said volute from said priming passage upwardly into said impeller during pump priming.

2. The pump of claim 1 wherein said ramp-like shoulder defines a surface extending at an angle inclined from the vertical within the range of about 5 to about 35.

3. The pump of claim 1 wherein:

a. said volute is arranged to discharge fluid into said separation chamber in a manner which will establish a fluid circulation in said separation chamber 5 along a predetermined flow path; and

b. said priming passage defines an inlet opening in said separation chamber facing away from the direction of fluid flow along said flow path whereby fluid entering said inlet opening from said flow path must at least partially reverse its direction of flow.

4. The pump of claim 3 wherein said inlet opening is located near said flow path and faces opposite the direction of fluid flow therealong whereby fluid entering said opening must execute a turn of about 180 to enter said inlet opening from said flow path.

5. A self-priming centrifugal pump capable of handling fluids containing solids. comprising:

a. a casing having walls defining suction and separation chambers;

b. a volute in said casing having a suction opening communicating with said suction chamber and a peripheral discharge opening communicating with said separation chamber;

c. an impeller journaled for rotation within said cas ing and positioned in said volute to draw fluid from said suction chamber through said suction opening and discharge it through said discharge opening into said separation chamber;

d. a priming passage communicating the lower region of said separation chamber and the lower region of said volute and defining a conduit of sufficiently large cross section to pass such solids as may be present in the fluid;

c. said conduit defining an inlet opening in said separation chamber and an outlet opening in said volute beneath said impeller; and

6. The pump of claim 5 wherein said ramp-like shoulder defines a surface extending at an angle inclined from the vertical within the range of about 5 to about 35.

7. The pump of claim 6 wherein said surface is inclined from the vertical within the range of about l5 to about 30.

8. The pump of claim 7 wherein said surface is inclined from the vertical within the range of about 20 to about 25.

9. The pump of claim 5 wherein said conduit defines a non-restricted turn intermediate said inlet and outlet openings which requires fluid passing therethrough to execute a turn of at least about 90 degrees before entering said volute.

10. The pump of claim 9 wherein:

a. said volute is arranged to discharge fluid into said separation chamber in a manner which will establish a fluid circulation in said separation chamber along a predetermined flow path; and

b. said faces inlet opening faces away from the direction of fluid flow along said flow path whereby fluid entering said inlet opening from said flow path must at least partially reverse its direction of flow.

11. The pump of claim 10 wherein said inlet opening is located near said flow path and faces opposite the direction of fluid flow therealong whereby fluid entering said opening must execute a turn of about I to enter said inlet opening from said flow path.

12. A self-priming centrifugal pump comprising a casing. a volute in said casing formed on a single spiral curve and having a suction opening a peripheral discharge opening in its upper portion and a priming opening in its lower portion. a rotatable impeller arranged in said volute to draw fluid through said suction opening into the center of said impeller and discharge the fluid through said discharge opening and an upstanding ramp-like shoulder formed in the lower portion of said volute adjacent to and downstream from said priming opening, said shoulder being arranged to direct fluid entering said volute through said priming opening upwardly into said impeller 13. A self-priming centrifugal pump according to claim 12 including a priming passage communicating with said priming opening and extending from said vo lute. said priming passage having an inlet which faces away from the direction of fluid circulation in said cas ing so that fluid entering said passage is caused to at least partially reverse its direction of flow.

14. A self-priming pump according to claim 13 in which said passage has a bend of about 90 between its inlet and said priming opening.

15. A self-priming centrifugal pump according to claim 13 in which said ramp-like shoulder defines a fluid deflecting surface inclined from the vertical at an angle in the range of from about to about 35.

16. A self-priming centrifugal pump comprising a casing having a suction inlet, a separation chamber and an outlet a volute in said casing formed on a single spiral curve and having a central suction opening, a pcripheral discharge opening in its upper portion and a priming opening in its lower portion, a rotatable impel Lit ler arranged in said volute to draw fluid through said suction opening into the center of said impeller and discharage the fluid through said discharge opening, a ramp-like shoulder formed in the lower portion of said volute and having a fluid deflecting surface adjacent to and downstream from said priming opening, said su rface being inclined to the vertical in order to direct fluid entering said volute through said priming opening upwardly into said impeller, and a priming passage communicating with said priming opening and extending from said volute through a bend of about said passage having an inlet facing away from the direction of fluid circulation in said separation chamber so that fluid entering said passage is caused to at least partially reverse its direction of flow.

17. A self-priming centrifugal pump comprising a casing, a volute in said casing having a suction opening, a peripheral discharge opening in its upper portion and a priming opening in its lower portion, a rotatable impeller arranged in said volute to draw fluid through said suction opening into the center of said impeller and dis charge such fluid through said discharge opening. and a conduit defining a priming passage communicating with said priming opening and having an inlet facing away from the direction of fluid circulation in said casing so that fluid entering said passage is caused to at least partially reverse its direction of flow.

18. The self-priming centrifugal pump of claim 17 wherein said passage is formed to require fluid passing thcrethrough to change its direction of flow.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,898,014

DATED August 5, 1975 INVENTOMS) David L. Meister, Edward L. Ohler and Frank D. I

II I I I I I I Makowski, Jr. It Is certified mat er or appears m the ab0vetdentifled patent and that sald Letters Patent are hereby con'ectad as shown below:

Col. 3 line 9 "centrifgual" should be --centrifugal. Col. 4 line 53, "rotoates" should be --rotates.

Claim 5 Col. 6, line 36 after "and" insert:

-f) an upstanding ramp-like shoulder formed in the lower region of said volute below said impeller at a position near to and downstream from said outlet opening for directing fluid entering said volute from said priming passage upwardly toward said impeller during pump priming.-

Col. 8, line 3 "charage" should be charge-.

Signed and Szaled this fourteenth Day Of October 1975 [SEAL] A trees I:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner of Parents and Trademarks 

1. A self-priming centrifugal pump capable of handling fluid containing solids, comprising: a. a casing; b. a volute in said casing and having a peripheral discharge opening in its upper portion, a priming opening in its lower portion, and a suction opening; c. a rotatable impeller positioned in said volute and being operable when rotated to draw fluid through said suction opening and discharge it through said discharge opening; d. a priming passage communicating with said priming opening and defining a conduit which requires fluid passing therethrough to change its direction of flow before entering said volute; and, e. an upstanding ramp-like shoulder formed in the lower region of said volute below said impeller adjacent to and downstream from said priming opening for directing fluid entering said volute from said priming passage upwardly into said impeller during pump priming.
 2. The pump of claim 1 wherein said ramp-like shoulder defines a surface extending at an angle inclined from the vertical within the range of about 5* to about 35*.
 3. The pump of claim 1 wherein: a. said volute is arranged to discharge fluid into said separation chamber in a manner which will establish a fluid circulation in said separation chamber along a predetermined flow path; and b. said priming passage defines an inlet opening in said separation chamber facing away from the direction of fluid flow along said flow path whereby fluid entering said inlet opening from said flow path must at least partially reverse its direction of flow.
 4. The pump of claim 3 wherein said inlet opening is located near said flow path and faces opposite the direction of fluid flow therealong whereby fluid entering said opening must execute a turn of about 180* to enter said inlet opening from said flow path.
 5. A self-priming centrifugal pump capable of handling fluids containing solids, comprising: a. a casing having walls defining suction and separation chambers; b. a volute in said casing having a suction opening communicating with said suction chamber and a peripheral discharge opening communicating with said separation chamber; c. an impeller journaled for rotation within said casing and positioned in said volute to draw fluid from said suction chamber through said suction opening and discharge it through said discharge opening into said separation chamber; d. a priming passage communicating the lower region of said separation chamber and the lower region of said volute and defining a conduit of sufficiently large cross section to pass such solids as may be present in the flUid; e. said conduit defining an inlet opening in said separation chamber and an outlet opening in said volute beneath said impeller; and
 6. The pump of claim 5 wherein said ramp-like shoulder defines a surface extending at an angle inclined from the vertical within the range of about 5* to about 35*.
 7. The pump of claim 6 wherein said surface is inclined from the vertical within the range of about 15* to about 30*.
 8. The pump of claim 7 wherein said surface is inclined from the vertical within the range of about 20* to about 25*.
 9. The pump of claim 5 wherein said conduit defines a non-restricted turn intermediate said inlet and outlet openings which requires fluid passing therethrough to execute a turn of at least about 90 degrees before entering said volute.
 10. The pump of claim 9 wherein: a. said volute is arranged to discharge fluid into said separation chamber in a manner which will establish a fluid circulation in said separation chamber along a predetermined flow path; and b. said faces inlet opening faces away from the direction of fluid flow along said flow path whereby fluid entering said inlet opening from said flow path must at least partially reverse its direction of flow.
 11. The pump of claim 10 wherein said inlet opening is located near said flow path and faces opposite the direction of fluid flow therealong whereby fluid entering said opening must execute a turn of about 180* to enter said inlet opening from said flow path.
 12. A self-priming centrifugal pump comprising a casing, a volute in said casing formed on a single spiral curve and having a suction opening, a peripheral discharge opening in its upper portion and a priming opening in its lower portion, a rotatable impeller arranged in said volute to draw fluid through said suction opening into the center of said impeller and discharge the fluid through said discharge opening, and an upstanding ramp-like shoulder formed in the lower portion of said volute adjacent to and downstream from said priming opening, said shoulder being arranged to direct fluid entering said volute through said priming opening upwardly into said impeller.
 13. A self-priming centrifugal pump according to claim 12 including a priming passage communicating with said priming opening and extending from said volute, said priming passage having an inlet which faces away from the direction of fluid circulation in said casing so that fluid entering said passage is caused to at least partially reverse its direction of flow.
 14. A self-priming pump according to claim 13 in which said passage has a bend of about 90* between its inlet and said priming opening.
 15. A self-priming centrifugal pump according to claim 13 in which said ramp-like shoulder defines a fluid deflecting surface inclined from the vertical at an angle in the range of from about 5* to about 35*.
 16. A self-priming centrifugal pump comprising a casing having a suction inlet, a separation chamber and an outlet, a volute in said casing formed on a single spiral curve and having a central suction opening, a peripheral discharge opening in its upper portion and a priming opening in its lower portion, a rotatable impeller arranged in said volute to draw fluid through said suction opening into the center of said impeller and discharage the fluid through said discharge opening, a ramp-like shoulder formed in the lower portion of said volute and having a fluid deflecting surface adjacent to and downstream from said priming opening, said surface being inclined to the vertical in order to direct fluid entering said volute through said priming opening upwardly into said impeller, and a priming passage communicating with said priming opening and extending from said volute through a bend of about 90*, said passage having an inlet facing away from the direction of fluid circulation in said separation chamber so that fluid entering said passage is caused to at least partially reverse its direction of flow.
 17. A self-priming centrifugal pump comprising a casing, a volute in said casing having a suction opening, a peripheral discharge opening in its upper portion and a priming opening in its lower portion, a rotatable impeller arranged in said volute to draw fluid through said suction opening into the center of said impeller and discharge such fluid through said discharge opening, and a conduit defining a priming passage communicating with said priming opening and having an inlet facing away from the direction of fluid circulation in said casing so that fluid entering said passage is caused to at least partially reverse its direction of flow.
 18. The self-priming centrifugal pump of claim 17 wherein said passage is formed to require fluid passing therethrough to change its direction of flow. 